Do you think House Butterwell would have owned their herds of cattle directly, or would they divide them up among their vassals? Or both? Given that fresh milk and cream aren’t very portable or storable without refrigeration, would they likely have invested in butter and cheese making operations as well? How important do you think the relative proximity to Kings Landing and it’s half a million potential customers was to Butterwell’s wealth?
Both.
The Butterwells would have owned cattle themselves, but only as many as they could run on the lands they owned directly. When it comes to land held in tenancy by their peasants, the cattle on those lands would most likely be owned by the tenants, although it wasn't unknown for peasants to rent livestock from their landlords.
Whether milk and cream were produced for market would have depended a lot on how close the farms were to towns and other commercial towns - if you were a day or less' travel away from a major population center, then selling milk and cream made economic sense.
However, it's also true that no self-respecting dairy farm would be without churns to make butter, cheese strainers and moulds to make cheese, and so forth. And the dairy farms of House Butterwell would both supply the Whitewalls "buttery," serve as a staple diet of Butterwell tenants, and would be sold on to nearby markets.
